Parys Mountain

Outline Geology and History

Notes on Parys Mountain (mainly geological) from Dr. Tim Barrett of
Ore Systems Consulting (OSC), 2000.

Snowdonia and Anglesey are two favourite areas geologically, historically and scenically, although I cannot profess to have any prolific knowledge in any of these areas. In 1999, a voluminous report on Parys Mountain was completed by OSC for the company that owns it, Anglesey Mining plc. An Abstract from that report is on the OSC web site. A paper summarizing the results has been submitted to the journal Economic Geology, and is under review (it was in fact published in 2001). In conjunction with this study, a reassessment of the existing paleontological data (graptolites and acritarchs) for the Parys Mountain area was completed by Dr. Mike Howe of Leicester University, a structural synthesis was completed by Dr. David James of Imperial College London and the first dating of the volcanic rocks was carried out by Dr. Randy Parrish of the British Geological Survey in Keyworth (he used to be at the University of British Columbia). Randy Parrish has shown that the rhyolites are at least 10 million years younger than the volcanism in Snowdonia, thus laying to rest the often-cited assumption that the volcanism in these two areas was coeval.

The massive sulphides at Parys Mountain, which were mainly discovered by drilling in the 1960s and 1970s, lie stratigraphically on top of mid-Ordovician shales, but are directly overlain by Llandovery (lowest Silurian) rhyolites and shales. The massive sulphides formed on the seafloor, as most VMS deposits do, probably from hydrothermal vents that deposited the metals from hot acidic fluids which reacted with cold oxidized seawater to cause precipitation of the metal sulphides. The massive sulphides at Parys Mountain form a series of small but high-grade lenses, more or less lying at the same stratigraphic contact. They are polymetallic, with good grades of Zn, Pb, Cu, Ag and Au. The main minerals are sphalerite, galena, chalcopyrite and pyrite. The non-ore minerals are quartz and carbonates. The footwall shales contain, in places, a series of sulphide-quartz-carbonate veins that represent the stockwork (or feeder veins) through which the fluids travelled en route to the surface. Some of the sulphide lenses contain very high Zn and Pb grades of up to 30% and 10%, respectively - these ores are locally known as bluestones. Some sulphide lenses at Parys Mountain comprise discrete beds of sulphide; these beds contain sulphide and shale clasts and therefore probably represent reworked clastic beds which formed when sulphide mounds on the seafloor partly disintegrated and shed debris laterally.

In the late 1700s and early 1800s, copper was mined from chalcopyrite-pyrite vein-like deposits utilizing an extensive series of shafts (now the collapsed open pits) and underground galleries. Although occurrences of bluestone were reported, it was not possible at that time to separate Pb and Zn from sulphide ores, so any such material that was encountered was discarded or sold for a low value. The origin of the copper veins has been long debated. Although they have been traditionally viewed as late tectonically controlled veins, partly because of the abundance of associated quartz (which forms the infamous 'white-rock' outcrops), it is also possible that the veins are related to the same mineralising system that produced the massive sulphides. Otherwise, one has to appeal to the 'lightning struck twice' hypothesis in order to account for the presence of two major mineralising systems almost side by side.

Unfortunately, there is not much information available on the geology of the old mines. However, a couple of excellent books were written in the sixties and seventies, which make interesting reading. These detail the history of mining and smelting at Parys Mountain, and its importance both to the industrial revolution and to the British economy in general. Much earlier in 1819, Michael Faraday visited the mine while taking a break from making scientific discoveries in Scotland and lucidly describes the operations in his book 'Travels in Wales'.

The old workings at Parys Mountain are now being explored and partly reopened though the efforts of a group of avid cavers and archaeologists led by Dr. David Jenkins of Bangor University.